A Look At Conference Call Services

In general, conference calls are designed for individuals who wish to make a call to have the option of multiple parties on the same line. A conference call can be set up in one of two ways, which includes a situation where only the caller speaks and the other parties simply listen or, in some cases, the listeners can also talk during the call. In short, a conference call can either be a one-sided call or one that involves the participation of all parties.

Conference calls can include the option of the caller actually making calls to the other participants or, in some cases individuals can call directly into a conference call by using a special telephone number. Most local telephone services offer three-way calling, which is a small variation of a typical conference call though the theme is the same. With three-way calling, multiple parties are permitted to use the same line during a conversation. This is ideal for phone customers who wish to speak with multiple family members or friends at the same time.

In the past, conference call services were reserved to only customers who subscribed to this option via their local telephone company. Today, however, conference call services are widely available online and are adding up to big savings on the telephone bills of many participants. In many cases, individuals often use video conferencing, which is the next level above conference calls.

A popular resource among most businesses, conference calls are used as a way of connecting business executives who are miles apart but need to speak to each other in an organized fashion. In addition, conference call services are very popular for use as a ‘party line.’ This is simply a specific line that allows multiple individuals to talk with one another via a specific telephone number. Although conference call services are most effective for business use, they are not completely out of the realm of possibilities for consumers and even entrepreneurs who work at home and need to connect with others.

Before deciding to sign up for conference call services from any one particular company, make sure that you take the time to shop around and compare rates from similar providers. Whether you are using conference call services that are provided through the telephone company or you wish to use those offered online, smart shopping applies to every aspect of life and this includes common everyday utilities.

About The Author

The author is a regular contributor to http://www.conferencecallsadvisor.com where more information about conference call services is available.

Surveillance Systems – A Legal Big Brother?

Surveillance systems, with all the sleek gadgetry have turned security into a high technology oriented big industry. Can you imagine what the small and concealable surveillance cameras can do to a country? US used satellite and aerial surveillance systems of the highest degree of sophistication, as recently as 2003-04 in Iraq and Afghanistan. This is the latest in international, state sponsored surveillance exercise of this magnitude. But this is not the first, or not in the least, the last. The 1965 demand of New York PD was honored by the Federal Government in 1969 and the first video cameras were installed in the New York City Municipal Building near City Hall.

Surveillance Systems for Public Security Use of surveillance systems soon became the norm in cities of political and strategic importance with the earlier analog video cameras being installed in public places. To name a few, Statue of Liberty and Lincoln Bridge were immediately covered by cameras for surveillance. Many crimes were either prevented or detected like never before. Still, the use of surveillance in America lags behind UK. The London Metro is the critical place where they have an extensive installation, which proved effective in the recent bombings.

State and private detectives use surveillance systems for their day to day work. The equipments they make use of are quite interesting as they are little marvels of technology. Body wearable spy cameras score on top of all of them. They come in different sizes and shapes making it easier to wear and conceal. They are mostly button shaped and most of them transmit their captures I instantaneously while some can store data for later retrieval.

Digital spy cameras which can be concealed anywhere inside a building sells for around $60, have 300’ wide angle and focal length of 6’. The smallest one measures 2CM x 2CM.

Phone bugs can’t be ignored at all. They have countless disguises like series transmitters, induction taps. They can be placed with the receiver of your phone, within the telephone box or even the near power socket. Don’t under estimate their capacities; they operate at 78 - 88MHz but professionals use range of 150-475 MHz to avoid detection. Price varies between $14.5 and $145.

‘Ear Spy’ helps hear across rooms through powerful devices connected to ear phones via long cords. Remote transmitter models have capabilities of 40-50GHz and various amplifications. Price ranges from $6 -$24.5

Home security is not quite different from public security system on certain counts. Some other popular equipments are GPS and GPS enabled cell phones are handy in tracing criminals and hostages.

http://www.searcharticles.net/article.cfm/id/16655

How plasma TVs work.

We're all familiar with the common cathode-ray tube (CRT) technology that has been the backbone of television for decades. Inside each CRT, an electron gun similar to a laser fires a negatively charged beam of electrons at groups of gas molecules (the pixels), which causes them to change color; producing the pictures we see everyday. These televisions work well and produce very crisp pictures, but they are notoriously bulky and heavy. This is because as the screen gets larger, the electron gun must be moved farther back so that it has a good angle to hit every pixel with its beam. Thus, the larger the screen, the deeper the TV.

Enter the plasma flat panel television. Arriving with the turn of the millennium, these televisions come in large, widescreen models that measure only 6 or 7 inches deep; a huge improvement over CRT. This dramatic change in shape results from individual transistor electrodes at each pixel. We no longer need the laser to hit every inch of the TV and, without the laser, manufacturers can eliminate most of the traditional bulk. The individual pixels in a plasma TV are composed of 3 fluorescent light cells: one red, one blue, and one green. The television produces pictures by varying the intensity of each cell to produce a unique color at every pixel without a laser. These lights give the television its name because they contain free flowing ions called plasma. The plasma, when hit with an electrical charge, produces light.

Traditional CRTs used the electron gun, or laser, to charge each pixel and create colored light. Plasma TVs instead have two sets of electrodes, one set running vertically and one set running horizontally. The horizontal set, which runs across the front of the screen, and the vertical electrodes, which run across the rear of the screen to form a grid like a checkerboard. The computer, by sending pecific charges through a single vertical and a single horizontal row, can color one pixel of plasma at a time. When the various sub-pixels are charged, the gas molecules inside release light particles called photons. The problem here is that photons are typically in the ultraviolet spectrum, and invisible to the naked eye. However, as they are released, they strike the surrounding surface of the cell. These surfaces have been specially coated with phosphors. A phosphor is a chemical that produces light, but only after being hit by another source of light. So, the invisible, ultraviolet photons strike the phosphor, creating a spectrum of light that our eyes can see.

Depending on how the three sub-pixels are charged, we may see a greener, redder, or bluer shade. These primary colors may combine to produce one of a million different colors. If you think about the basics of this technology, each light cell is a miniature fluorescent light. This produces the brilliant, flicker-less picture we see when we watch a plasma television. Now you understand the technology behind the plasma flat panel television.

http://www.searcharticles.net/article.cfm/id/17443

Metal Detector Buying Guide

If you were one person who loves adventure in the form of treasure hunting, then a metal detector would be one of your necessary equipments. Be it out of sheer hobby or more on the professional level, you really would not perform well treasure hunting if you do not have a good metal detector to explore with.

A metal detector is one sophisticated device that leads you toward gold mines. A good metal detector would sure find some small treasures buried in sands, in soils, and in other obscured places. A good, wholesome fun is sure to be delivered by this ingenious device.

If you are planning to buy a metal detector to join the treasure-hunting fun, here are some tips to guide you so that youll get the best deal possible. Metal detectors can be pricey, so youve got to invest your money well.

1. Determine where you are going to use the device at

There are certain features needed for certain places the device is going to be used for. If you intend to use it on the beach, the terrain wont be that rigorous. But you are going to need waterproof protection. If you wanted to use it more rigidly, then you should opt for the one that is especially made for all kinds of terrains.

2. Estimate for how long you are going to use the detector

The hours of operation a metal detector is determined by its model. Some are made to work for as long as 24 hours a day. Some cant perform as much. This is decided by the battery life of the device you are going to purchase. Also decide if you rather have extra batteries or if you prefer to get a charging kit.

3. Know your budget for it

As stated earlier, metal detectors could really be expensive. You can cut on cost if you cut on features. Try to look for the best bargain by enumerating the features you are going to need against the one that you can leave off. Also, be on the look out for brands that are big on features but small in price. Now thats a real good deal.

4. Research the latest technologies

As with other electronic equipments, innovations seems to always happen overnight. If you are going to buy a metal detector today, try to know that technologies are currently employed. You do not want to be behind the technology line, right?

5. Get some professional advice

Get an advice from people who are the known authorities in the field of metal detectors. Although families and friends are good sources of information, they might not be as updated as the experts are. If you wanted to get the best device possible, get first-hand information from the experts themselves.

6. Consider durability

Youve got to buy a metal detector that could withstand normal wear and tear longer than its competitors. The longer the metal detector can serve you, the better your return on investment will be.

7. Consider accuracy

Of course, the metal detector has to perform, and very well at that. It should deliver accurate detection. It should also carry out the task real well. It should spot treasures at once, and in a very precise manner. This is important because this is its main job out on the field. If it cannot detect small jewelry, as it should, then it is not worth buying, no matter how a good deal it seemed to be.

8. Get the most adaptable one

This can be determined by the metal detectors ability to be upgraded, when possible. Get the one that has a lot of devices that can be attached or added to it to make it work better. Do not settle for the detector that is relatively stagnant. If the detectors features can be enhanced further, the better buy it is.

These points could serve as your guide in buying a new metal detector. The more advantage points the particular type of detector you are eyeing for has, the better buy it is. Just make sure that you cash on the every feature you paid for. Buying a metal detector is a real decision. Buying that one is an even harder job. Follow these steps and you are sure to be guided accordingly.

About The Author

For more great metal detector related articles and resources check out http://123metaldetectors.info

How To Buy Used Metal Detectors

Once you have decided to buy a metal detector, you have to decide whether you want to buy a brand new instrument or a second hand model. If budget is a constraint, a used metal detector will be a better option. If you are wondering how to buy a used metal detector, here are some useful tips, which will guide you in your purchase.

What are the types of metal detectors?

There are various kinds of metal detectors available in the market, meant for different purposes. The most easily available ones are gold detectors, coin and jewel detectors, relic hunting detectors, beach-hunting detectors, and underwater metal detectors.

What are the factors that I should take into account before buying a used metal detector? Consider the following points before your purchase:

· The amount of usage of the detector: If you plan to use it for a new hobby, choose an instrument offering various features.

· Location of usage: Decide if you plan to use it on the beach, below water or in the forest.

· Decide on who will use it: For a family use, there are detectors with adjustable arm cups and separate pouches to place the electronic box.

· Budget: Compromise on your budget to some extent so that you can buy the one meeting a majority of your needs.

Where should I buy?

A secondhand metal detector is a good bargain for your money. However, ensure you buy it from a reputed dealer rather than from an advertisement in your local newspaper. A reputed dealer will give you a money back guarantee or allow you to exchange your old one. This is because they test and then sell the detectors. If you choose to buy from any other source than buy from a good dealer or from a person you trust.

What are the prices of the used metal detectors?

All-purpose metal detectors carry a price tag of $275 to $700. Underwater metal detectors come at a price of about $500. The secondhand accessories come at the price of $200. But the only drawback is, there is no manufacturer’s warranty but at least buying from a good dealer will provide some safety.

http://www.searcharticles.net/article.cfm/id/16650

Recent Progress on Pb-free Electronics at IBM

Lead-containing solders have been used extensively in microelectronic interconnect structures between various packaging levels. Since the RoHS enforcement date of July 1, 2006 by the EU is rapidly approaching, the transition to Pb-free solders is accelerating in the electronics industry.1

At IBM, considerable R&D efforts on Pb-free solders technology were conducted well before the RoHS and WEEE Directives were enacted by the EU, as noticed in the early publication and patent literature.2-5 An example of the application of Pb-free technology within IBM is the usage of Bi-Sn eutectic solder for low-temperature wave soldering of plated through holes (PTH) in advanced multilayer PCBs since the early 1970s.6-7 IBM’s early work led to the National Center for Manufacturing Sciences (NCMS) Pb-free solder project, the first industry-wide joint effort to search for Pb-free solders. When the final report of the NCMS Pb-free project was published in 1997, it was dedicated to the memory of Roger Wild (former IBMer) for his pioneering work on Pb-free soldering technology at IBM.8

Fundamentals

Recently, worldwide R&D efforts on Pb-free solder alloys have identified several promising candidates for different soldering applications, as listed in Table 1. Two major Pb-containing solders, eutectic 63Sn-37Pb and Pb-rich solders used for SMT and flip chip or C4 (controlled collapse chip connection) solder bumps are also included in Table 1. It should be noted that the compositions of most Pb-free solder candidates are Sn-rich solders that contain more than 90% Sn. This suggests that the physical, chemical, and mechanical properties of the proposed Pb-free solders are heavily influenced by the properties of pure Sn, as opposed to eutectic Sn-Pb, which microstructure consists of a mixture of Sn-rich and Pb-rich lamellar phases. Pure Sn is polymorphic, capable of existing as three different crystal structures (∝, β, γ) depending on temperature and pressure.9 Since the white tin phase (β-Sn) has a body-centered tetragonal (BCT) crystal structure in contrast to the face-centered cubic (FCC) structure of Pb, the physical and mechanical properties of white tin are less isotropic and more difficult to mechanically deform in comparison to Pb. Since the white Sn crystal is optically birefringent, polarized light microscopy can be used to distinguish the orientation of β-Sn dendrite structures in Pb-free solders.10

The melting point of most Pb-free commercial solders is within the range of 208º to 227ºC, which is about 30ºC higher than the melting point of eutectic Sn-Pb at 183ºC. The higher melting point has serious implications on the performance of packaging materials and assembly processes, and can affect the integrity and/or reliability of Pb-free microelectronic packages. Another important issue associated with the proposed Pb-free solders is the difficulty of maintaining solder melting-point hierarchy, which has been well established with Pb-containing solders. With the proposed Pb-free solders, the maximum differentiation in solder melting temperatures between any two package levels is less than 30ºC. The process control, during successive multiple soldering processes with Pb-free solders, becomes more challenging and its impact on solder joint and package reliability is not yet well understood.

Among the several Pb-free solder candidates, the near-ternary eutectic Sn-Ag-Cu (SAC) alloy compositions with melting temperatures around 217ºC is the consensus solder alloy for SMT card assembly, including BGA solder joints, while the eutectic Sn-Cu or Sn-Ag-Cu alloy is a promising choice for Pb-free flip chip applications.11-14

Pb-free Ceramic Ball Grid Arrays

IBM’s development team conducted evaluations, using a number of criteria, in the early phase. Sn-3.8Ag-0.7Cu (SAC in wt. %) was selected based on its excellent wettability to I/O pads, stable microstructure of solder joints, superior ball shear strength, and the excellent thermal-mechanical fatigue (TMF) behavior of assembled modules subjected to accelerated thermal cycling.15 Further work with SAC confirmed that it was an optimal replacement alloy for Sn-Pb CBGAs for both 1.27- and 1.00-mm pitch.16

Reliability evaluation was performed with co-fired multilayer 9211 (alumina-based) ceramic test vehicles (32.5-mm square to 42.5-mm square, with thickness ranging from 1.50 to 3.70 mm). Five ceramic modules were attached by using SAC solder balls to a FR4 test card (6S4P, 229 × 279 mm). A typical cross section of a CBGA solder joint connecting a ceramic module to a Cu pad on a PCB (as-joined) is shown in Figure 1. Standard accelerated thermal cycle (ATC) testing (from 0° to 100°C, with 2 cycles/hr.) was used on the assemblies with a periodic, 4-point resistance measurement. An increase in resistance of 10 Ω or greater was used to define a failure. Table 2 summarizes the MTF data of Pb-free CBGA modules in comparison with the Sn-Pb, IBM dual-melt structure as a benchmark. The N50s were calculated by fitting a lognormal distribution to the fatigue data. All of the SAC alloy test vehicles produced well-controlled fatigue life as indicated by the small sigma. The SAC alloy has almost twice the fatigue life of the dual alloy Sn/Pb structure, for the same form factor. The full melting of SAC balls during assembly causes a smaller joint height with the SAC structure than the Sn-Pb structure. In spite of the unfavorable ratio of joint heights (1:2), the SAC alloy demonstrates the superior fatigue properties over the Sn-Pb for the same form factor. The TMF performance of SAC, combined with the relative lack of complexity in manufacturing these assemblies, makes the Pb-free CBGA technology a winner on all fronts.

Pb-free PBGA

The transition to Pb-free soldering technology has been juxtaposed on other market forces driving PBGA packaging toward high-performance, higher I/O, and high-reliability applications. The confluence of all these factors in time has made the development of Pb-free organic packaging technology a challenging watershed to cross. The higher reflow temperatures associated with Pb-free packaging technology have mandated the use of new organic packaging materials with improved water absorption characteristics, reduced CTE’s, and improved thermal stability. Higher reflow temperatures and the accompanying evolution toward higher I/O and larger chip sizes result in higher package stresses and place more stringent requirements on interfacial adhesion within electronic package structures. The stresses fostering package delamination derive from CTE mismatch, from increased steam pressurization within the packages, due to the presence of absorbed moisture, and in the case of flip chip packaging from the full melting of the C4 bumps within the underfill encapsulation. The choice of solderable surface finish and its bearing on solder joint fragility is a significant concern across the industry at this time. In this regard, IBM has chosen Cu pad structures as the surface finish for most of its packaging requirements. Successful Pb-free packaging, capable of passing stringent reliability testing, has mandated improved materials and significant interfacial adhesion augmentation.

Wire-Bond, PBGA Packaging. IBM has produced Pb-free wire-bond PBGA packages since early 2002 in both glob-top and overmolded configurations, and expects to be qualified to produce Pb-free and halogen-free products by the end of 2005. Package sizes range from 15 to 45 mm, with 1.27- and 1.0-mm BGA pitch. To achieve these results, new glob-top and overmold materials were developed in close working relationships with materials suppliers. The new materials have improved adhesion and moisture absorption characteristics, together with improved wire sweep processing characteristics capable of accommodating complex wire-bond trajectories.

Flip Chip, PBGA Packaging. Flip chip packaging is associated with a broader range of packaging challenges in achieving a successful transition to Pb-free soldering technology. New BLM (Ball Limiting Metallurgy or UBM) structures are required to accommodate the use of high Sn, Pb-free, solders. The full melting, Pb-free, C4 structures provide for higher internal package stresses during reflow because the bump volume increase associated with the melting process is constrained by the underfill encapsulant. The use of new underfill materials was required to meet the Pb-free reflow requirements. In addition, new thermal solutions and lower CTE laminate materials were incorporated to improve package reliability as the simultaneous evolutionary transition was made toward higher-performance modules. Package qualifications have been successful with the new organic material set and solder alloys. Flip chip PBGA products are now qualified from 15 to 42.5 mm for both 225- and 200-µm C4 pitch with 1.27- and 1.0-mm BGA pitch. Future extension to 55-mm package size is anticipated in the near future.

Pb-free Flip Chip Solder Plating

IBM is developing a Pb-free C4 solder bumping process for flip chip applications in response to customer requirements and environmental regulations. IBM has been plating C4 solder (97Pb-3Sn) for more than 9 years using a TiW/CrCu/Cu BLM structure.

The development approach for Pb-free C4 plating incorporated several key ground rules. A multifunctional team including development, process, equipment, quality, reliability and product engineering, analysis, research, and manufacturing was formed. Minimal changes to the existing successful BLM structure were desired. The new Pb-free process is designed to maximize use of the existing production line capital infrastructure and process for ease of manufacturing.

Two key components of the Pb-free C4 structure are the BLM and the C4 metallurgy. It is well known that the Pb-free, Sn-rich solders are highly reactive and, therefore, require a robust barrier metallurgy in the BLM to withstand aggressive high-temperature-storage (HTS) and electromigration requirements. IBM has been using a Ni barrier for other products, and chose to incorporate Ni as the barrier for Pb-free C4. For the solder metallurgy, we evaluated Sn-Cu as well as Sn-Ag-Cu (SAC) constructions. The evaluation criteria included ease of manufacturing, quality and reliability. From a reliability standpoint, the SAC and Sn-Cu solders performed equivalently. With our current tool set, the greater ease of manufacturing and better bump quality of Sn-Cu resulted in our choice of Sn-Cu over SAC. The construction IBM subsequently qualified for production is a TiW/CrCu/Cu/Ni BLM with Sn-Cu solder metallurgy, as shown in Figure 2. The first Pb-free C4 application that IBM qualified and put into production is a chip-on-flex one. We completed a production line qualification as well as a reliability assessment. There were no C4 fails in deep-thermal-cycling (DTC), HTS, low-temperature-storage (LTS), temperature/humidity and bias (THB), high-temperature-operating-life (HTOL) or ATC stresses. Electromigration performance of this Pb-free C4 was better than expected, between eutectic Sn-Pb solder and 97Pb-3Sn solder in performance. This application is now in production at IBM.

The key challenge for the future will be to provide Pb-free C4 bumps at 150-µm pitch and below. IBM is currently developing solutions for that application space. IBM has developed, qualified, and is in production for Pb-free C4 bumps. Qualification for additional applications is in progress, with a good prognosis for continued success.

Pb-free Wafer Bumping

Multiple transitions are presently occurring in the wafer bumping industry. Among these are increased wafer size from 200 to 300 mm, decreased bump size and pitch, alloy change from eutectic Pb-Sn to Pb-free, and the continued growth of flip chip vs. wire bond. At the same time, the demand for lower costs, finer pitch, and higher quality is unrelenting. In September 2004, IBM and SUSS MicroTech announced a new wafer bumping technology called “C4NP,” short for Controlled Collapse Chip Connect New Process, to address these challenges.

C4NP grew out of earlier work at IBM Research in which a new injection-molded solder (IMS) process produced advanced thermal interfaces. It was quickly realized that IMS could readily be applied for making solder interconnect structures. A parallel process, C4NP fills molten solder into small cavities in a bump template that matches the I/O footprint of a silicon wafer. After the solder is solidified, an inspection step ensures that all the cavities are properly filled. Separately, wafers are prepared with the appropriate BLM compatible with the desired solder alloy. The last step actually bumps the wafer, namely aligning the filled bump template and joining it in mirror-image fashion to the silicon wafer above the solder reflow temperature. This transfers the solder volumes from the cavities to the wafer. The bump templates are reusable, keeping costs low. Several advantages can be listed for C4NP:

* Alloy flexibility including multicomponent Pb-free alloys;
* No volume change from deposition to final bump; extendible to fine bump size and pitch;
* Same tool set for both 200-mm and 300-mm wafers;
* Low material costs in comparison to paste, preform, or chemical solution;
* Optimal yields by bump template inspection before transfer to wafer;
* Rapid turn-around time by prefilling bump templates ahead of wafer completion;
* Efficient solder usage for environmental and economic benefits;
* Extendability to finer pitch;
* Process simplicity similar to stencil printing.


As seen in Table 3, a comparison of other established bumping processes explains why C4NP is creating excitement in the industry. Considering the challenges ahead, the timing of a new bumping technology that combines high-end capabilities with low-end costs may be considered auspicious. More than 40 years ago, IBM first introduced C4 technology. C4NP, as the latest version, should provide a cost-effective solution to meet present and future Pb-free wafer bumping needs.


http://smt.pennnet.com/display_article/233628/95/ARTCL/none/none/Recent-Progress-on-Pb-free-Electronics-at-IBM/

Tin Whiskers: What’s the Risk?

The adoption of lead-free technology, driven by legislative requirements and market forces, has involved significant changes in materials, processes, and supply chain in the electronics industry. The changeover has raised a number of concerns for the electronics industry in ensuring product reliability while maintaining reasonable costs.

As the EU’s Restriction of Hazardous Substances (RoHS) deadline of July 2006 approaches, the industry has predominantly settled on tin-silver-copper as the replacement for conventional tin-lead solder. To comply with RoHS, part manufacturers have sought lead-free finishes to replace the traditionally used tin-lead finishes. The finish selection is important in providing corrosion resistance, good solderability, and durable solder joints. Currently, selected lead-free finishes include pure tin, tin-bismuth, tin-silver, and tin copper, and to a lesser extent nickel-palladium-gold and nickel-gold. Due to its low cost and compatibility with existing solders, the pure tin and tin-rich lead-free alloys have been adopted by a significant portion of electronic part manufacturers.

Particularly among the high-reliability product community (space, military, and high-end computer servers and storage), the adoption of pure tin and tin-rich lead-free finishes has raised a reliability issue pertaining to the formation of conductive whiskers. In 2002, CALCE in collaboration with individuals from NASA, Boeing, Raytheon, and the U.S. Navy, issued an alert outlining the potential risks imposed by the selection of pure tin and tin-rich lead-free alloys. The concern prompted several industrial groups, including CALCE, to examine the phenomenon of tin whisker formation, the effectiveness of various risk mitigation strategies, and methods for quantifying the risk presented by the use of these finishes in electronic products. As a gauge to the level of concern, iNEMI has three working groups on tin-whisker-related issues, and CALCE maintains a mailing list of individuals (engineers, scientists, and managers) from more than 60 organizations who have participated in a weekly forum on whisker-related issues that has been ongoing for almost 3 years.

While the phenomenon of tin whiskers has regained considerable attention during the past 3 years, key questions related to the tin whisker growth mechanisms remain. Major issues include:

1. Wide proliferation of pure tin and tin-rich lead-free finished electronic components in the market.
2. No consensus on tin whisker growth mechanism.
3. No recognized method of accelerating whisker formation.
4. No guaranteed method of avoiding tin whisker growth.

As a result, the industry continues to be challenged in answering the basic question of quantifying the risk presented by this wide adoption of pure tin and tin-rich lead-free alloys.

What Are Tin Whiskers?

For the uninitiated, tin whiskers are spontaneous growths of tin from tin-finished surfaces such as those shown in Figure 1. These growths can be in the form of nodules, needle-like filaments, or odd-shaped eruptions. The needle-like, straight whiskers are a chief concern, because there is a higher chance for bridging adjacent electrical conductors. Such bridging may result in excessive current leakage and/or electrical shorts in the electronic system.

The phenomenon of tin whisker formation was first reported in the 1950s by the Bell Telephone Co.1 To date, a wide variation in the growth characteristics, including length (0.3 to 10 mm), growth rate, and incubation time (days to years), has been reported. While leadframe parts have received significant attention, other parts including connectors, cover plates, and fasteners must also be considered at risk if finished with whisker-prone materials such as tin, zinc, and cadmium

For tin whiskers to form, tin must migrate to the whisker site. However, it is unclear if the tin moves along the surface or from under the surface. It may very well come from both routes. Although the exact cause of whisker growth is not completely understood, it is widely believed that compressive stress within the tin finish influences tin whisker growth.2,3 Stresses within the finish can arise from:

• Intermetallic compound formation between the plating material and substrate, resulting in compressive stress within the plating;
• Mismatches in coefficient of thermal expansion (CTE) of the tin-based plating material and substrate, or underlayer;
• Presence of residual stress from electroplating process itself;
• Extrinsic compressive stress, such mechanical bending and forming.

Regardless of the surface finishing technology, namely electroplating, hot air leveling, or immersion tin, tin whiskers have been observed to form. However, electroplated surfaces are considered to be a higher risk, since the electroplating process involves various parameters affecting the level of stress in the plating deposit. Key parameters include plating chemistry, organic additives, current density of the plating bath, and bath contamination. While the effect of those plating parameters on tin whisker growth is not fully understood, some consider mechanical handling, which the finished surface may be subjected to after the plating, to be even more critical.

Intermetallic formation between the tin plating and substrate material appears to be a contributing factor. In the case of tin-plated copper substrates a dominant diffusion species, tin, migrates relatively quickly through grain boundaries and dislocations, or more slowly through the bulk, resulting in Cu6Sn5 intermetallic (IMC) formation at room temperature. The scallop-like Cu6Sn5 IMC with density lower than that of copper is known to give rise to compressive stress within the finish. The usage of underlayer, such as nickel, is a possible means to reduce the effect of intermetallic formation on copper leadframes, because tin-nickel IMC grows slower than tin-copper IMC. In fact, some part manufacturers have adopted a nickel underlayer and its beneficial influence for retarding whisker growth has been reported in various studies.4-7 In the case of tin-plated alloy 42 substrate, compressive stress within the finish may arise under thermal cycling, due to a larger CTE mismatch with pure tin.

Review of the Electronic Component Market

For the past 3 years, CALCE has tracked the selection of finishes by more than 100 part suppliers and has observed a continued increase in selection of pure tin and tin-rich lead-free alloys for lead finishes (Figure 3). Tin-rich finishes have been a preferred choice, mainly due to the low cost, good processability, good corrosion resistance, and compatibility both with conventional tin-lead and lead-free solders.8 The finishes offered by a variety of part manufacturers include pure tin, tin-bismuth, tin-copper, and tin-silver.

http://smt.pennnet.com/display_article/233627/95/ARTCL/none/none/Tin-Whiskers:-What%E2%80%99s-the-Risk?/